The present invention relates to an optical digital transmission system monitoring system, in particular, it relates to such a system which does not request an interstitial monitoring line.
In a digital transmission system which includes repeaters, an error rate measurement in each repeater section is needed to monitor the operation of the repeaters, and to locate the fault position of the transmission line.
Conventionally, there have been two monitoring systems. The first is an out-service method which measures the line by sending a particular signal in the line after stopping the communication service. The second is an in-service method which measures the line by interposing a particular test signal on a commercial signal without stopping the communication service.
In an out-service method, a return path between an upward line and a downward line is provided in a repeater for test purposes, so that a test signal from a transmission station returns to the transmission station through the repeater. The out-service method has the advantage that it does not require an interstitial cable for test purposes.
In an in-service method, a parity bit of a communication signal is handled so that it indicates error information, which is returned to a transmission station through an interstitial line (CCITT report COM X VIII No. 59-E 1977 August). However, an in-service method has the disadvantage that particular repeaters, for an interstitial line itself, are required for a long transmission line. Therefore, a prior in-service method is not cost effective and operationally reliable.
A prior in-service method is described in accordance with FIGS. 1 through 3.
FIG. 1(a) shows a signal which is transmitted to a line. The numerals 11, 12, 13, 14 and 15 are signal blocks, and numerals 21, 22, 23, and 24 are parity bits. It is assumed that an even parity law is followed, that is, if the number of marks (1) in the block 11 is even, the parity 21 is "0", and if that number is odd, the parity 21 is "1". Assuming that the parity bit 21 is included in the block 11, the number of marks in the block 11 which includes the parity 21 is always even. Therefore, when the number of the signal pulses of FIG. 1(a) is divided into 1/2 by using a flip-flop, the output of the flip-flop relating the parity bits 21, 22, 23, 24, . . . , is fixed to "1" or "0", so long as no error occurs. FIG. 1(b) shows the output of the flip-flop. In FIG. 1(b), the numeral 3 shows the halved pulses divided from the block 11, and the signal "1" and/or "0" occurrs at random. A signal 4 is located at the bit position of the parity 21, and it is assumed that the signal 4 is "1". When an error occurs in a transmission line at the bit position 5, the number of marks of the block 13, which includes the parity 23, becomes odd (not even). Therefore, the output of the flip-flop is inverted to zero at location 41 which relates to the parity bit 23. Then, if no error occurs in the block 14, the output of the flip-flop does not change, and therefore, the output is still zero at location 42. Therefore, it should be noted that when there is a bit error and an even parity state is broken, the output of the flip-flop is inverted. The flip-flop is of course inverted if the parity bit is inverted purposely.
FIG. 1(c) shows the DC (direct current) component of the signal of FIG. 1(b) which is processed by a low-pass filter. As apparent in FIG. 1(c), the potential of the signal changes at the location where a bit error occurs. Accordingly, a terminal station may recognize the presence of a bit error by receiving the DC component through an interstitial cable.
When there are a plurality of repeaters coupled in series on a transmission line, the particular repeater must be identified. A prior identification of a repeater is now described. As shown in FIG. 2, an output of the flip-flop 8 is coupled with the interstitial cable 9 through the bandpass filter 10. It is assumed that a bandpass filter 10 of each repeater has its own center frequency f.sub.0. A terminal station transmits a signal which has the frequency 2f.sub.0 and an odd parity as shown in FIG. 3(a). The output of the flip-flop 8 is the low frequency signal 6' of the frequency f.sub.0 as shown in FIG. 3(b). When a bit error occurs at location 5, the number of marks in the block becomes even, and therefore, the output of the flip-flop does not change its potential as shown by numeral 7' in FIG. 3(b). Accordingly, a bit error in a repeater is recognized in a terminal station by sensing a period of an odd parity of a return signal.
However, the prior in-service method for monitoring repeaters has the disadvantage that an interstitial cable for merely monitoring is required. When a cable is long, an interstitial cable itself requires repeaters, and therefore, a prior system is not practical in economical consideration and operational reliability. Further, the prior art has a disadvantage in regard to its power supply. It should be noted that repeaters in a transmission line are coupled in series with a power supply line, and are supplied with constant current, so that repeaters operate with small supply current. However, as each repeater has a plurality of repeater circuits or regenerators for upwards circuits and downward circuit, the supply current from a terminal station is a multiple of the current which is required for each regenerator. When a number of repeater circuits or regenerators in a repeater is small (2 or 3 like a prior coaxial cable system), the multiple supply current raises no problem. However, in case of an optical fiber system in which more than 6 regenerators are mounted in each repeater, the supply current must be large, and that large supply current creates some problems in the system design.
Accordingly, the prior system in FIGS. 1 and 2, in which a DC component is transmitted over a plurality of repeater sections and a DC coupling exists relating to a power supply, is not advantageous in an optical transmission system in which each repeater has a plurality of regenerators, and each repeater is supplied with a constant current.